An active matrix type liquid crystal display having a pixel matrix circuit and a driver circuit comprising a thin film transistor (TFT) formed on a substrate having an insulating property is receiving attention. The liquid crystal display is utilized as a display device of a size of from 0.5 to 20 inches.
At the present time, in order to realize a liquid crystal display capable of conducting highly minute display, a TFT having a crystalline semiconductor film, such as polysilicon, as an active layer is receiving attention. The TFT having a crystalline semiconductor film as an active layer has a higher operation rate and a higher driving capability than a TFT having an amorphous semiconductor layer as an active layer, but involves a problem in that unevenness in electric characteristics of the respective TFT is large.
One of the policy of development of a liquid crystal display is to develop one having a large area. When a liquid crystal display has a large area, the pixel matrix circuit to be a pixel display part also has a large area, and thus a source wiring and a gate wiring arranged in a matrix form become long to increase a wiring resistance. Furthermore, in order to cope with the demand of high minuteness, the wiring must be thin to make the increase in wiring resistance conspicuous. The source wiring and the gate wiring are connected to a TFT for the respective pixels, and increase in number of pixels brings about a problem of increase in parasitic capacity. In the liquid crystal display, the gate wiring and the gate electrode are generally formed as unified, and the delay of the gate signal becomes conspicuous with the display panel having a large area.
Accordingly, the lower the resistivity of the material of the gate electrode wiring is, the thinner and longer the gate wiring can be, and thus a display panel of a large area can be produced. While Al, Ta and Ti have been used as the material of the gate electrode wiring, Al has been frequently used since Al has the lowest resistivity among them and is a metal capable of being subjected to anodic oxidation. The heat resistance of Al can be increased by forming an anodic oxidation film. However, even at a process temperature of from 300 to 400° C., whiskers and hillocks are formed, and the wiring suffers deformation and diffusion into an insulating film and an active layer, which become causes of operation failure of the TFT and deterioration of the TFT characteristics.
Furthermore, in order to conduct anodic oxidation, an electrode and a wiring to be subjected to anodic oxidation must be connected to a voltage supplying line, and after completing the anodic oxidation treatment, the voltage supplying line and the unnecessary connection part to the voltage supplying line must be removed by etching. Therefore, in order to produce a thin film transistor by using the anodic oxidation treatment, a space for forming the voltage supplying line and an etching margin are necessary, which prevent integration of the circuit.
In recent years, a high mobility is demanded for a TFT, and it is the major trend that a crystalline semiconductor film having a higher mobility than an amorphous semiconductor film is used as an active layer of the TFT. A conventional TFT is produced in the manner, which will be described below.
An amorphous silicon film is formed on an insulating substrate, and the amorphous silicon film is subjected to a crystallization treatment by heating or irradiation with laser light, to form a polysilicon film (polycrystalline silicon film). After patterning the polysilicon film in a desired shape, a gate insulating film and a layer of a material for forming a gate electrode are accumulated thereon, and a gate electrode is formed by patterning them. An impurity giving a conductivity is then selectively introduced into the polysilicon film to form an impurity region to be a source region and a drain region. After an interlayer insulating film is then accumulated, and a contact hole is formed to expose the source region and the drain region, a metallic film is formed and patterned to form a metallic wiring in contact with the source region and the drain region. Thus, the production process of the TFT is finished.
In the conventional method described above, after the semiconductor film having an amorphous substance is formed, the gate insulating film is formed after conducting some steps (such as the crystallization step and the patterning step).
Therefore, before forming the gate insulating film, the surface of the crystalline semiconductor film to be an active layer is contaminated or oxidized by an impurity in the air (oxygen and water) or an impurity formed in the steps before the formation of the gate insulating film. When the gate insulating film is formed on the crystalline semiconductor film having the contaminated or oxidized surface, the active layer, particularly the interface characteristics between a channel forming region and the gate insulating film, is deteriorated, to become a cause of unevenness and lowering of the electric characteristics of the TFT.